A method is proposed for determining the metabolites of radioactive 5-hydroxytryptophan and of endogenous tryptophan in the rat brain. The compounds are separated by thin-layer chromatography and made visible with p-dimethylaminocinnamaldehyde (DACA) reagent. The conditions studied are those which allow the use of DACA reagent for the quantitative determination of tryptophan metabolites in brain tissue. Amount of 5 ng of serotonin and 20 ng of tryptophan can be determined by densitometry.